All crude oil contains impurities which contribute to corrosion, heat exchanger fouling, furnace coking, catalyst deactivation and product degradation in refining and other processes. These contaminants are broadly classified as salts, bottom sediment and water, solids, and metals. The amounts of these impurities vary depending upon the particular crude. Generally, crude oil salt content ranges between about 3 and 200 pounds per 1000 barrels.
Brines present in crude include predominently sodium chloride with lesser amounts of magnesium chloride and calcium chloride being present. Chloride salts are the source of highly corrosive HCL, which is severely damaging to refinery tower trays and other equipment. Additionally, the carbonate and sulfate salts may be present in the crude in sufficient quantities to promote crude preheat exchanger scaling.
Solids other than salts are equally harmful. For example sand, clay, volcanic ash, drilling mud, rust, iron sulfite, metal and scale may be present and can cause fouling, plugging, abrasion, erosion and residual product contamination. As a contributor to waste and pollution, sediment stabilizes emulsions in the form of oil wetted solids, and can carry significant quantities of oil into the waste recovery system. Organic materials such as benzene may be transferred to the wash water stream causing wastewater discharge problems. Current and proposed limitations on refinery wastewater discharge exists. Specific limitations on benzene content are particularly relevant to desalter operations since desalter effluent brine has been identified as a major source (60 to 80%) of benzene in refinery wastewater.
Desalting is, as the name implies, adapted to remove primarily inorganic salts from the crude prior to refining. The desalting step is provided by adding and mixing with the crude a few volume percentages of fresh water to contact the brine and salts.
In crude oil desalting, a water-in-oil emulsion is intentionally formed with the water admitted being on the order of about 4 to 40 volume percent based on the crude oil. Water is added to the crude and mixed intimately to transfer impurities in the crude to the water phase. Separation of the phases occurs due to coalescence of the small water droplets into progressively larger droplets and eventual gravity separation of the oil and underlying water phase.
Demulsification agents are added, usually upstream from the desalter to help in providing maximum mixing of the oil and water phases in the desalter. Known demulsifying agents include water soluble salts, Twitchell reagents, sulfonated glycerides, sulfonated oils, acetylated castor oils, ethoxylated phenolformaldehyde resins, a variety of polyester materials, and many other commercially available compounds. In addition to demulsifiers, other materials may be fed to the crude. For example, U.S. Pat. No. 5,080,779 discloses the addition of water soluble chelants to control iron.
Desalters are also commonly provided with electrodes to impart an electric field in the desalter. This serves to polarize the dispersed water droplets. The so formed dipole droplets exert an attractive force between oppositely charged poles with the increased attractive force increasing the speed of water droplet coalescence to from 10 to 100 fold. The water droplets also move quickly in the electrical field, thus promoting random collisions that further enhance coalescence.
Upon separation of the phases from the water-in-oil emulsion, the crude is commonly drawn off the top of the desalter and sent to the fractionator tower in crude units or other refinery processes. The water phase containing water soluble metal salt compounds, other contaminants, and sediment is discharged as effluent. It is a high benzene content in this effluent water stream that the present invention controls.
Desalters are typically employed in tandem arrangement to improve salt removal efficacy. Commonly, in such designs, crude oil from the resolved emulsion in the upstream, first desalter is used as crude feed to the downstream second desalter. Fresh wash water is added to the crude stream fed to the second desalter, with water phase bottoms effluent from the second desalter being fed back as makeup water, mixed with the crude fed to the first desalter.
Typical desalters are provided with heat imparting means and pressure control means. Typically, desalter temperatures are maintained at 90.degree. to 150.degree. C. Heat lowers the viscosity of the continuous phase (oil) thereby speeding the settlement of the coalesced water droplets as governed by Stokes law. It also increases the ability of bulk oil to dissolve certain organic emulsion stabilizers that may have been added or are naturally occurring in the crude.
Desalter pressure is kept high enough to prevent crude oil or water vaporization. Desalter pressures at operating temperatures should be about 20 psi above the crude oil or water vapor pressure, whichever is higher.
Emulsion breakers, also called the demulsifiers, are usually fed to the crude so as to modify the stabilizer film formed initially at the oil/water interface. These emulsion breakers are surfactants that migrate to the interface allowing droplets of water or oil to coalesce more readily. These demulsifiers reduce residence time required for good separation of oil and water.
Due to the advantage of heat in aiding separation, in a conventional system the crude oil fed to the first stage desalter is preheated prior to mixing with the effluent water from the second stage in feeding to the desalter unit. Thus, in a conventional two-stage desalter system both the first and second stage of the desalter train are operated at elevated temperatures.